1. Field of the Invention
The present invention relates generally to a variable grating diffractive light modulator, and more particularly, to a variable grating diffractive light modulator, which actuates lower reflecting plates of a lower reflecting array by a piezoelectric actuating method, and includes an upper reflecting array that is spaced apart from the lower reflecting array and attached to a lower surface of a substrate that is made of a light transmissive material and covers the lower reflecting array, so that the light modulator reflects incident light transmitted through the substrate and thereby produces diffractive light.
2. Description of the Related Art
Generally, an optical signal processing technology has advantages in that a great amount of data is quickly processed in a parallel manner unlike a conventional digital information processing technology in which it is impossible to process a great amount of data in real time. Studies have been conducted on the design and production of a binary phase only filter, an optical logic gate, a light amplifier, an image processing technique, an optical device, and a light modulator using a spatial light modulation theory. The spatial light modulator is applied to optical memory, optical display device, printer, optical interconnection, and hologram fields, and studies have been conducted to develop a display device employing it.
The spatial light modulator is embodied by a reflective deformable grating light modulator 10 as shown in FIG. 1. The modulator 10 is disclosed in U.S. Pat. No. 5,311,360 by Bloom et al. The modulator 10 includes a plurality of reflective deformable ribbons 18, which have reflective surface parts, are suspended on an upper part of a silicon substrate 16, and are spaced apart from each other at regular intervals. An insulating layer 11 is deposited on the silicon substrate 16. Subsequently, a sacrificial silicon dioxide film 12 and a low-stress silicon nitride film 14 are deposited. The nitride film 14 is patterned by the ribbons 18, and a portion of the silicon dioxide film 12 is etched, thereby maintaining the ribbons 18 on the oxide spacer layer 12 by a nitride frame 20. In order to modulate light having a single wavelength of λo, the modulator is designed so that the height difference of the ribbon 18 and oxide spacer 12 is equal to a multiple of λo/2.
Limited by a vertical distance between a reflective surface 22 of each ribbon 18 and a reflective surface of the substrate 16, a grating amplitude of the modulator 10 is controlled by applying voltage between the ribbon 18 (the reflective surface 22 of the ribbon 18 acting as a first electrode) and the substrate 16 (a conductive layer 24 formed on a lower side of the substrate 16 to act as a second electrode). In an undeformed state of the light modulator with no voltage application, the grating amplitude is λo/2 while a total round-trip path difference between light beams reflected from the ribbon and substrate is λo. Thus, a phase of reflected light is reinforced. Accordingly, in the undeformed state, the modulator 10 acts as a plane mirror when it reflects incident light. In FIG. 2, reference numeral 20 denotes the incident light reflected by the modulator 10 in the undeformed state.
When a proper voltage is applied between the ribbon 18 and substrate 16, the electrostatic force enables the ribbon 18 to move downward toward the surface of the substrate 16. At this time, the grating amplitude is changed to λo/4. The total round-trip path difference is a half of a wavelength, and light reflected from the deformed ribbon 18 and light reflected from the substrate 16 are subjected to destructive interference. The modulator diffracts incident light 26 using the interference. In FIG. 3, reference numerals 28 and 30 denote light beams diffracted in +/− diffractive modes (D+1, D−1) in the deformed state, respectively.
It has been proven that sticking of the ribbon 18 to the substrate 16 is a common problem of the light modulator 10 during a wet process applied to form a space under the ribbon 18 and during operation of the modulator 10. There are various methods of reducing the sticking: lyophilization, an OTS single layer treatment, use of a hard ribbon and/or a tightened nitride film gained by shortening the ribbon, a method of roughing or wrinkling one or both surfaces of two facing surfaces, a method of forming a reverse rail on the lower part of the ribbon, and a method of changing the chemical properties of the surfaces. In a solid-state sensor and actuator workshop held in June, 1994 at the Hilton Head Island in Scotland, prevention of sticking was reported, which is accomplished by reducing the contact area by forming a reverse rail on the lower part of a bridge and by employing a rough polysilicon layer as disclosed in “a process of finely treating the surface of a deformable grating light valve for high resolution display devices” suggested by Sandeyas, et al., and “a grating light valve for high resolution display devices”, suggested by Apte et al.
Moreover, Apte et al. found that mechanical operation of the modulator 10 has a characteristic such that deformation of the ribbon 18 as a function of voltage forms hysteresis. The hysteresis is theoretically based on the fact that an electrostatic attractive force between the ribbon 18 and substrate 16 is a nonlinear function of the deformation, whereas hardness of the ribbon 18 is a substantially linear function of a resilient force by tension.
U.S. Pat. No. 5,311,360 by Bloom et al. discloses a latching feature which gives a modulator 10 advantages of an active matrix design without the need for active components. Additionally, Bloom et al. describes that this feature is valuable in low power applications where efficient use of available power is very important. However, Bloom et al. discloses the addition of small ridges below ribbons 18 to reduce a contact area, thereby reducing the sticking problem. However, since the substrate of the modulator 10 is used as an optical surface, a process of adding the small ridges to the surface is complicated in that a reflective element of the substrate 16 must be smooth so as to have high reflectance and must be positioned on a planar surface of the ribbon 18.
Typical display devices are formed in 2-D arrays of pixels. Discontinuous images formed by a plurality of pixels are integrated by user's eyes, thereby forming an aggregate image of pixels constituting a whole image. Unfortunately, prices of such a display device are high because the pixels are overlapped to form a complete array, so the production cost of each pixel is duplicated. The display device comprising pixels is exemplified by televisions or computer systems. Their pixels may be formed by an LCD device or a CRT device.
Accordingly, there is required a diffractive grating light valve capable of reducing or removing the sticking between the reflective element and the substrate without a complicated surface treatment adopted to reduce the sticking.
As well, a display device is required, which reduces the number of pixels to reduce production costs without reducing image quality while designing a system.
To satisfy the above requirements, a conventional improved technology is proposed in Korean Pat. Application No. 10-2000-7014798, entitled “method and device for modulating incident light beam to form 2-D image”, by Silicon Light Machines Inc.
In the “method and device for modulating the incident light beam to form the 2-D image”, the diffractive grating light valve includes a plurality of elongate elements each having a reflective surface. The elongate elements are arranged on an upper side of a substrate so that they are parallel to each other, have support ends, and their reflective surfaces lie in array (GLV array). The elongate elements form groups according to display elements. The groups alternately apply voltage to the substrate, resulting in deformation of the elements. The almost planar center portion of each deformed elongate element is parallel to and spaced from the center portion of the undeformed element by a predetermined distance which is set to ⅓–¼ of the distance between the undeformed reflective surface and the substrate. Thus, the deformed elongate elements are prevented from coming into contact with the surface of the substrate. Sticking between the elongate elements and the substrate is prevented by preventing contact between the elements and substrate. Additionally, the predetermined distance between each deformed elongate element and the substrate is limited so as to prevent hysteresis causing deformation of the elongate elements.
FIG. 4 is a side sectional view of an elongate element 100 of a GLV in an undeformed state according to a conventional improved technology. In FIG. 4, the elongate element 100 is suspended above a surface of a substrate (including constitution layers) by ends thereof. In FIG. 5, reference numeral 102 denotes an air space.
FIG. 5 is a side sectional view of a deformed elongate element 100 of the GLV. FIG. 5 shows that the deformed elongate element 100 is maintained in the suspended state thereof to be spaced from the surface of the substrate adjacent therebeneath. This is in contrast to the conventional modulator of FIGS. 1 to 3. Contact between the elongate element 100 and the surface of the substrate is prevented, thereby avoiding the disadvantages of conventional modulators. However, the elongate element 100 is apt to sag in the deformed state. The reason is that the elongate element 100 is uniformly subjected to an electrostatic attractive force acting toward the substrate in directions perpendicular to a longitudinal direction thereof, whereas tension of the elongate element 100 acts along the length of the elongate element 100. Therefore, the reflective surface of the elongate element is not planar but curvilinear.
However, the center part 102 of the elongate element 100 (FIG. 8) is almost planar, making the contrast ratio of diffracted light, gained by only the center part 102 of the elongate element 100, desirable. The substantially planar center part 102 has a length that is ⅓ of a distance between post holes 110. Hence, when the distance between the post holes 110 is 75 μm, the almost planar center part 102 is about 25 μm long.
FIG. 6 is a front view of the display element 200 in which the deformed elongate elements 100 are alternately arranged. FIG. 6 is a view taken in the direction of the arrows along the line B–B′ of FIG. 5. The elongate ribbons 100 which are not removed are maintained at desired positions by an applied bias voltage. Deformation of the moving elongate ribbons 100 is achieved by alternate applications of operation voltages through the conductive layer 106 to the elongate elements 100. A vertical distance (d1) is almost constant to the almost planar center part 102 (FIG. 5), thereby limiting the grating amplitude of the GLV. The grating amplitude (d1) may be controlled by adjusting an operation voltage on the operated elongate elements 100. This results in precision tuning of the GLV in an optimum contrast ratio.
As for diffractive incident light having a single wavelength (λ1), it is preferable that the GLV has a grating width (d1) that is ¼ (λo/4) of the wavelength of incident light to assure a maximum contrast ratio in an image to be displayed. However, the grating width (d1) requires only a round trip distance that is the same as the sum of a half of the wavelength (λ1) and the whole number of the wavelength (λ1) (i.e. d1=λ1/4, 3λ1/4, 5λ1/4, . . . , Nλ1/2+λ1/4).
Referring to FIG. 6, the lower side of each elongate element 100 is spaced upward from the substrate by a distance of d2. Accordingly, the elongate elements 100 do not come into contact with the substrate during operation of the GLV. This results in avoidance of the sticking problems between the reflective ribbons and the substrate occurring in conventional modulators.
However, the conventional technology inevitably requires a gap between micromirrors to actuate the micromirrors with a ribbon shape. As the gap increases, a fill factor is reduced with respective to the same ribbon width. Hence, a maximum quantity of light which is diffracted to 0th or ±1 st order becomes small, thus reducing a dynamic range of the light modulator.
According to the conventional technology, the light modulator has various pitches, according to adapted areas, including printing or displaying areas. The light modulator must minimize the gap between the micromirrors under a given pitch. In the case of a light modulator having a small pitch, a high fill factor is required to assure a sufficient modulation dynamic range, thereby a small gap is required. However, it is very difficult to form a small gap. Further, as the gap is reduced, the capacity of the device is deteriorated.
Further, the conventional technology is problematic in that the diffraction efficiency is lowered, and the uniformity of the output light of all pixels is thus lowered, when the actuating distance of three or four micromirrors provided in one pixel to be simultaneously actuated is not accurately regulated.